Syntheses and Redox Properties of Carboxylate-Ligated Hexanuclear Ce(IV) Clusters and Their Photoinduced Homolysis of the Ce(IV)-Ligand Covalent Bond.

ABSTRACT

Oxo- and hydroxo-bridged hexanuclear Ce(IV) clusters surrounded by 12 carboxylate ligands, Ce6O4(OH)4(O2CR)12(L)n (R = 2,6-Me2-4-MeOC6H2 (1a), 2,6-Me2-4-tBuC6H2 (1b), 2,4,6-Me3C6H2 (1c), 2,6-Me2C6H3 (1d), 2,6-Me2-4-FC6H2 (1e), 2,6-Me2-4-ClC6H2 (1f), 9-anthracenyl (1g), and CH2tBu (1h), L = H2O or RCO2H), were synthesized by treating Ce(OtBu)4 with the corresponding carboxylic acids (2-3 equiv.) in acetone or toluene, and the molecular structures of 1d and 1g were clarified by X-ray diffraction studies. UV-vis analyses of the clusters showed broad absorption corresponding to the ligand-to-metal charge transfer (LMCT) in the ultraviolet A (315-400 nm) to blue light region; density functional theory (DFT) studies of the simplified Ce(IV) and related Zr(IV) clusters, M6O4(OH)4(O2CR)12 (M = Ce, Zr, R = Ph, Me), revealed that the low-lying vacant 4f-orbitals of the Ce(IV) were responsible for absorption in the ultraviolet A to blue light region. Irradiation of blue LED light to 1a-f under an argon atmosphere resulted in the formation of 7-methylisobenzofuran-1(3H)-one (2a-f), which involved the following four steps: photoinduced homolysis of the Ce(IV)-OCOR bond, intramolecular hydrogen atom transfer to generate the corresponding benzyl radical, oxidation to the benzyl cation, and intramolecular cyclization. Cyclic voltammetry of cerium clusters 1a-f having 2,6-dimethyl-4-substituted arylcarboxylate ligands showed electrochemically irreversible redox waves in the range of -0.79 to -0.38 V (vs [Cp2Fe]+/Cp2Fe for E1/2). The one-electron-reduced Ce(III)Ce(IV)5 clusters 3a-h were isolated by reducing 1a-h with Cp*2Co to give [Cp*2Co][Ce6O4(OH)4(O2CR)12(thf)n] (3a-h); cluster 3d was the first structurally determined hexanuclear cerium cluster containing a [Ce6O4(OH)4]11+ core.